As electronic equipment is reduced in size and increased in packaging density, multilayer wiring boards having a plurality of boards laminated to one another have prevailed as printed circuit boards. In any multilayer wiring board, it is necessary to electrically connect conductive layers of upper and lower laminated boards with each other. For this purpose, via holes reaching a conductive layer of a lower layer are formed in an insulating layer of the multilayer wiring board, and conductive plating is performed on the inside of each via hole so as to electrically connect the conductive layers of the upper and lower laminated boards.
As via holes are made finer, a high power CO2 laser or a UV laser using harmonics of YAG is used for forming the via holes. In addition, high-speed machining can be attained by scanning with a laser beam using a beam scan optical system comprising a combination of steerable mirrors and fθ lenses. However, as the holes are made smaller in diameter or smaller in pitch, more severe accuracy with respect to a machining position has been requested. Alignment marks provided in the four corners of a printed circuit board are often used as references for positioning the printed circuit board. Specifically, the printed circuit board is mounted on an XY table. An image of each alignment mark is picked up from above by a camera or the like. The center of the alignment mark is obtained by image processing. Then, machining is performed while a displacement of each point on the board is calculated and corrected. However, the alignment mark may be altered from their predetermined shape due to a failure in etching, a dust, etc.
In order to reduce a position error caused by the defective shapes of alignment marks, there has been proposed a technique in which one alignment mark consists of a plurality of small marks (JP-A-2001-291095 (Page 3, FIG. 1)).
There has been another technique in which hole marks are disposed concentrically (JP-A-S61-125712 (FIG. 8)).
However, in JP-A-2001-291095, the center positions of the small marks differ from one to another. Accordingly, due to influence of aberration of a camera lens or the like, it is difficult to make the position error of a reference point, for example, smaller than 5 μm by use of a combination of an inexpensive camera and an inexpensive camera lens.
On the other hand, in JP-A-S61-125712, there is little influence of aberration of a camera lens or the like. However, the center position of the hole marks is obtained from a part of the hole marks disposed concentrically. Accordingly, due to influence of a failure in etching, a dust, etc., it is difficult to make the position error of a reference position, for example, smaller than 5 μm.